Such utility knives, sometimes called carton knives, may be used for the opening of cartons and similar packages and are intended to prevent accidents by providing that the knife blade is automatically pulled back into its protected position by the force of the return member when the knife is not operating. In order to advance the knife blade into its operating position, the known knives have an outwardly protruding handle piece which can be moved by one's thumb. This way, the operator can push the knife blade outward, which, however, has the disadvantage that upon frequent use during the day, one's thumb becomes tired very rapidly. Consequently, the knife blade is either locked in the operating position by manipulation of the slide or utility knives are used which are not equipped with the safety device described.